The invention relates to an antenna formed in strip transmission line, the antenna comprising a plurality of conductive strip antenna elements distributed over an antenna aperture which extends in each of two mutually perpendicular directions, and feeding means for supplying energy to the elements. The feeding means comprise an elongate primary strip transmission line feeder for applying energy from a port coupled thereto and further comprise a plurality of secondary strip transmission line feeders coupled to the primary feeder at intervals therealong. Each of the secondary feeders is coupled at one end to the primary feeder, extends away therefrom and has a respective plurality of the antenna elements coupled to it at intervals therealong.
The invention relates particularly but not exclusively to such an antenna having two, three or four ports and radiation patterns which are respectively associated with the supply of energy to the antenna at the respective ports and which have respective single main lobes with substantially different respective angular orientations.
For convenience, references in this specification to the operation of an antenna generally relate (as above) to the supply of power to the antenna, i.e. to use of the antenna for transmission, but might equally well relate mutatis mutandis to the derivation of power from the antenna, i.e. to use of the antenna for reception.
The invention is especially applicable to antennae having a large number of antenna elements, for example a hundred elements and possibly many hundreds of elements. Such an antenna may be used to produce a main lobe having a fairly narrow 3 dB beamwidth, for example in the range 1-20.degree.. An example is an antenna in a Doppler navigation system for an aircraft, for which it may be desirable to have a beamwidth of approximately 5.degree..
An antenna as set forth in the opening paragraph, intended for a Doppler navigation system, is known from the paper "A Printed Circuit Antenna for a Doppler Navigator" by M. Scorer and B. J. Adams, IEE Colloquium on Advances in Printed Antenna Design and Manufacture, London, 18th Feb., 1982, pages 7/1 to 7/8. The means used in that antenna for coupling energy from the primary feeder to the secondary feeders are strip-conductor T-junctions with a 2-section transformer in each secondary feeder adjacent the junction. T-junctions have also been used in other multi-port antennae employing strip transmission line means for supplying power to the radiating elements: see for example U.K. Patent Application 2 107 936 B and the paper "A Printed Antenna/Randome (Radant) for Airborne Doppler Navigational Radar" by T. W. Bazire, R. Croydon and R.H.J. Cary, International Conference on Antennas for Aircraft and Spacecraft, June, 3-5, 1975, London, pages 35-40. However, the proportion of power supplied at such a junction cannot always be accurately controlled; furthermore, there is effectively a lower limit to the proportion of power supplied to the side arm of such a junction (in this application of the junction, from the primary to the secondary feeder). This problem becomes particularly acute if the antenna has a large number of elements, for example several hundred, with a substantial number of secondary feeders, and/or if the operating frequency is relatively high, for example in K.sub.a band (e.g. around 30 GHz), in which case the widths of strip transmission line conductors with typical thicknesses of substrate and for typical impedances tend to be fairly large in terms of wavelength but small in absolute terms, effectively limiting the practicable aspect ratios of the lines at a junction. Furthermore, with regard to controlling the proportion of the power available to each antenna element that is actually radiated thereby, in order to tailor the illumination across the antenna aperture, there is an analogous limitation on the range of radiation conductances that can be obtained by varying the widths of the antenna elements.
The antenna described in the above-mentioned paper by Scorer and Adams is intended for operation at about 13 GHz and uses nine secondary feeders each with a large number of antenna elements coupled thereto, there being a small taper along the primary feeder. This probably lies close to the limit of what is practicable using T-junctions. It is desirable to provide an alternative arrangement which can enable a larger number of secondary feeders to be used if desired, for example to achieve a narrower beam width in the general direction of the primary feeder, and/or which is more suitable for use at higher frequencies.
The problem is especially severe if, as in the above-mentioned known antennae, the antenna has at least two ports with associated respective radiation patterns. The general nature of the problem is as follows. To achieve low sidelobe levels, a well-tailored distribution of power across the antenna aperture must be achieved. To obtain two radiation patterns whose main lobes have different angular orientations, it is necessary to supply energy from ports on opposite sides of the antenna aperture to an array of elements distributed across the aperture. The further an element is from the port at which energy is being supplied, the less energy will generally be available to it, since a substantial proportion of the energy supplied at the port will already have been radiated by elements closer to that port. It is therefore desirable that elements which are relatively close to a port should only accept a small proportion of the available power, in order to leave some power for those elements which are relatively remote from the port. However, elements which are relatively close to one of two ports are relatively remote from the other of those ports, and, if designed to accept only a small proportion of available power in order not to use too much power when energy is supplied at the nearby port, will radiate a fairly negligible amount of power when energy is supplied to the other, relatively remote part. The variation with distance from a port in the proportion of power supplied to elements therefore needs to be carefully selected to achieve a suitable compromise, and the values actually achieved in a constructed antenna should preferably be close to the theoretical design values if repeated modifications are to be avoided.